Where the Vine Roots Live - Understanding Vineyard Soils - page 151

Box 4.10 (continued)

If ET data are used, FC at bud burst must be measured or estimated as
described earlier. The SWD isassumedtobezerothenanditssubsequent
development tracked using weekly rainfall and ET data. For example, if the
effective rainfall in the first week of bud burst is P 1 millimeters and the actual ET is
ET 1 , the SWD at week's end is given by

(B4.10.1)

SWDPET
1

=−

1 1

Rainfall is measured and actual ET estimated, as described in boxes 4.5 and
4.7,respectively.Notethatif P 1 > ET 1 , there is surplus water that can be lost as
drainage or runoff; conversely, if P 1 < ET 1 , a deficit develops. For the second week
of bud burst when rainfall is P 2 and ET is ET 2 , the SWD is calculated as

SWD WD P T
2

= + ( − ) (B4.10.2)

1 2 2

hiscalculationcanberepeatedforsubsequentweeksandtheresults
accumulated through to harvest. As shown in figure B4.10.1, the trigger point
for irrigation occurs when the threshold SWD is reached and sufficient water is
then applied to keep the SWD within the desired range, depending on the stage of
growth.
he ET method is widely used in California's Central Valley region where
the growing season weather is very reliable. Over 120 meteorological stations,
distributed around the state as part of the California Irrigation Management
InformationSystem(CIMIS; www.cimis.water.ca.gov) ,collectweatherdata
minute by minute and calculate and store daily values. These “raw” values
are transmitted via the Internet to a central computer once a day, where they
arequality-checkedandvaluesof PET calculated for the individual stations.
WinegrowerswhoareregisteredasCIMISuserscanaccessdatarelevanttotheir
district and adjust daily PET to actual ET usingacropcoeicient(seebox4.7).
Irrigation scheduling is then determined by applying the soil water balance
equation(e.g.,equationB4.10.1)onasequentialdailybasis.heCIMISwebsite
has an example of such a calculation.
Daily PET values are available from a similar but less coordinated system
ofweatherstationsinsouthernAustralia(seemea.com.au/soil-plants-climate/
weather/free-weather-data).

emerging(igure4.18).hegaspressurejustsuicienttoforcesapfromthecut
endisassumedtobeequalbutoppositetothe LW P . For stem water potential,
a leaf close to the trunk is enclosed for at least an hour to prevent transpiration
andtoallowittoequilibratewiththerestofthevine.heleafisthenplucked
and measured as for LW P .Stemwaterpotentialhastheadvantageofbeing
independent of the leaf 's position on the trunk or main branch.